The objectives of this project are the development of synthetic routes to two classes of medium ring natural products exemplified by eremantholide A and the complex diterpenoid taxusin. No efficient synthetic routes to these complex substances are presently known, yet these substances or related derivatives possess marked and promising antitumor activity in a variety of tumor cell lines. The proposed synthetic route to eremantholide A is convergent and features the preparation and coupling of two key optically active fragments. The proposed route to taxusin features the development of a two-carbon ring expansion, and the preparation of the bridgehead olefin by an aldol-dehydration sequence.